Stand on a stepladder beneath the left side of the motor head, the part of the opener mounted to the ceiling, and locate the limit dials.
Turn the "Down" limit dial counterclockwise a full revolution to add 2 inches of downward travel, turn it counterclockwise to remove travel.
Turn the "Up" limit dial clockwise a full revolution to add 2 inches of upward travel. This adjustment is useful if the door doesn't fully open.
Test the adjustments by opening and closing the garage door. Climb back up on the stepladder if further changes are necessary.